1. Field of the Invention
The present invention relates to starch-based corrugating adhesives which contain hemicellulose. More particularly, the invention relates to high speed corrugating adhesives of the carrier, no-carrier or carrier-no-carrier type which are prepared by incorporating hemicellulose in the adhesive formulation to obtain improved green strength and water resistance.
2. Description of Related Art
Adhesives used in manufacturing corrugated board are usually comprised of starch, caustic, a boron containing compound and, where water resistance is needed, a waterproofing resin. The main binder of corrugating paper is the starch which is gelatinized in the corrugating process as it penetrates the paper fiber. The other components, namely, the caustic, boron containing compound, and waterproofing resin are auxiliary agents which modify the basic properties of the starch.
Caustic, usually in the form of sodium hydroxide, directly affects the gelatinization point temperature of the starch. Gelatinization point temperature is often referred to as gel point or gel temperature and is reduced from its natural value of 165xc2x0 F. (74xc2x0 C.) for unmodified corn starch to a temperature in the range from about 138xc2x0 F. (59xc2x0 C.) to about 152xc2x0 F. (67xc2x0 C.) by the addition of caustic. Caustic also enhances penetration of the gelatinized starch into the surface paper fibers allowing for better bonding.
The boron containing compounds perform multiple tasks. The most important function is developing the adhesive tackiness that is crucial in the formation of the xe2x80x9cgreen bondxe2x80x9d. The green bond is the bond which holds the components of the corrugated board together until final heat curing of the adhesive system. It relies solely on the viscosity of the adhesive to maintain the integrity of the product during curing. Corrugating machine speeds therefore are limited by the rate of viscosity increase (i.e., rate of green bond formation) in the bond line between the liner and the corrugated medium. The oxygen atoms attached to the boron form stronger bonds, sometimes called boron bridges, between the starch and cellulose hydroxyl groups. Boron containing compounds also act as buffering agents in the presence of caustic and help maintain the viscosity stability of the adhesive paste.
It is known that polyhydroxy compounds other than starch, such as polyvinyl alcohol, will respond with the boron containing compounds in a manner similar to that with starch. Boron containing compounds and polyvinyl alcohol are believed to interact synergistically to form strong bonds. Like other polyhydroxy compounds, polyvinyl alcohol in the presence of starch will develop adhesive tackiness faster in the presence of boron, which means that the corrugator can operate at higher machine speeds.
Water proof or water resistant resins are considered to be an optional component of adhesives, and although some corrugating plants may not need them, most plants are prepared to use them when needed. Many types of thermo-setting resins are suitable. The most common of them are derived from urea-formaldehyde, ketone-formaldehyde or melamine-formaldehyde. When heat and pressure is applied to them in a corrugating machine, they will condense to form an excellent water resistant network. When polyvinyl alcohol is added to adhesives containing these resins, it participates in cross-linking which will improve the water resistant bonds.
In the corrugating process, adhesive is commonly applied to the tips of the flutes of a corrugated medium. Then a noncorrugated flat paper liner is applied against the adhesive coated flutes as they pass between a corrugating roll and a pressure roll. The resulting product has the corrugating medium on one side and a flat liner on the other side and is called a single-faced portion. The single-faced portion may be used xe2x80x9cas isxe2x80x9d (called a xe2x80x9csingle facerxe2x80x9d board) or adhesive may be applied to the flute tips of the single-faced portion and a second flat sheet can be applied in the same manner as the first in what is called a xe2x80x9cdouble-facerxe2x80x9d or a xe2x80x9cdouble-backerxe2x80x9d operation. The second liner sheet is treated with heat and reduced pressure (relative to the pressure used to make a single-faced portion) immediately following contact with the adhesive.
Starch-based adhesives which can be of the carrier, no-carrier and carrier-no-carrier type are commonly used in processes for manufacturing corrugated paper board. In carrier type adhesives, a portion of the starch (or dextrin) forms a carrier, often known as the gelatinized phase, which suspends the balance of the starch which is in an ungelatinized state. Under conditions of heat and pressure, the ungelatinized starch is rapidly hydrated and gelatinized to increase quickly the viscosity and adhesivity of the adhesive composition. In no-carrier type adhesives, all of the starch is slightly cooked or swollen with heat and caustic soda for viscosity. Finally, carrier-no-carrier type adhesives have a portion of the starch which forms a carrier and is responsible for about one half of the viscosity and the remaining viscosity is obtained by slightly swelling the uncooked starch.
One of the most important properties of a corrugating adhesive is green strength. Green strength is the characteristic that holds the paper together until the full strength of an adhesive develops. The green strength contributed by conventional starch adhesives is fairly low, but the addition of substances like polyvinyl alcohol improve the property as noted above. Increased levels of green strength, sometimes called early tack, allow corrugating machine operators to increase the speed of their machines without sacrificing the quality of the product.
Synthetic polymers that affect green strength tend to be expensive and their prices follow the volatility of the petroleum market. These polymers also are perceived by the public as less desirable than natural products. Moreover, the use of such polymers can require additional processing steps which adds expense for the corrugator.
According to U.S. Pat. No. 4,941,922, fibers can be added to starch-based corrugating adhesives to enhance adhesion and dispersion and yield improved adhesive characteristics including increased waterproofness, dry strength, viscosity and adhesiveness. Suitable sources of fiber are described as cellulosic fiber including wood, paper, cotton and rayon; fibers of synthetic origin including Nylon, polyester, polypropylene, Lycra Spandex, Vyrene, Vinyon, Dynel, Saran, Creslan, Acrylic, polyethylene, Teflon, tetrafluoroethylene, glass, Corlon and metallic fibers. Fibers generated during the wheat or corn milling processes are also said to be suitable. According to the patent, the fibers typically are suspended initially in the uncooked starch portion, suspended with the carrier portion, suspended prior to the carrier-no-carrier (or no-carrier) swell or added to the final adhesive mixture prior to storage or use. There is no experimental data in the patent, however, describing the use of fibers generated during the corn milling process and the patent does not disclose the extraction of hemicellulose from fibers of any kind. Furthermore, the fibers must be present in an amount from about 2.2 to about 22 pounds of fiber per 100 pounds of corrugating adhesive and have a diameter of between about 0.0005 inches and about 0.02 inches and a length of between about 0.025 inches and about 0.25 inches.
It has now been found that hemicellulose, a natural and readily available component of corn kernels, hulls and other plant materials, can be used to enhance green strength in starch-based corrugating adhesives. Hemicellulose is a pentoglycan which can exist in various forms depending upon the plant source.
It has also now been found that hemicellulose can be extracted in situ from corn fiber during the cooking phase in the manufacture of carrier type and carrier-no-carrier type corrugating adhesives. Corn fiber produced by the corn wet milling industry and having a particle size in the adhesive, or in a carrier phase, of less than about 0.005 inch is preferred.
In the present specification and claims, all parts and percentages are by weight unless otherwise specified, and the term xe2x80x9cparts of adhesivexe2x80x9d is used to refer to the total weight of adhesive, including water, starch and all chemicals. The term xe2x80x9cparts of carrier phasexe2x80x9d is used to refer to the total weight of the carrier phase, including water, starch and all chemicals. The terms xe2x80x9ctotal starchxe2x80x9d and xe2x80x9ctotal carbohydratexe2x80x9d refer to the total cumulative weight of unmodified starch, modified starch and dextrin present, i.e. all of the carbohydrates.
The corrugating adhesive composition of the invention is a carrier, no-carrier or carrier-no-carrier type, starch-based corrugating adhesive which contains from about 0.1% to about 20.0%, preferably from about 0.3% to about 10.0%, hemicellulose by weight based on total starch. The hemicellulose can be derived from various plant materials including wood and agricultural products, and a preferred source of hemicellulose is corn. Hemicellulose also can be slightly hydrolyzed to improve viscosity. A partially hydrolyzed hemicellulose is available under the trade designation CELLACE from NSK, Tokyo, Japan.
Hemicellulose can be added to the primary or secondary mixer in the preparation of a carrier type corrugating adhesive or it can be added at any stage in the preparation of a no-carrier or a carrier-no-carrier type adhesive. In the preferred embodiment, hemicellulose is extracted in situ from corn fiber which has been added to the primary mixer in the preparation of a carrier type or carrier-no-carrier type corrugating adhesive.
According to the preferred embodiment, the carrier type corrugating adhesive of the invention is made by the following process:
1. Water, unmodified starch, and/or modified starch and/or dextrin, and corn fiber are mixed in a primary mixer and heated for at least about 1 minute and preferably from about 1 minute to about 25 minutes at a temperature of from about 115xc2x0 F. (46xc2x0 C.) to about 180xc2x0 F. (82xc2x0 C.);
2. An aqueous solution of caustic is added to attain a pH from about 10 to about 14, preferably from about 12 to about 14, and mixing is continued for at least about 10 minutes and preferably from about 10 to about 40 minutes;
3. Additional water is added and mixing is continued for at least about 1 minute and preferably from about 1 to about 15 minutes, most preferably from about 5 to about 8 minutes, to make a carrier phase;
4. A secondary mixer is charged with water, heated to from about 70xc2x0 F. (21xc2x0 C.) to about 105xc2x0 F. (41xc2x0 C.), preferably from about 90xc2x0 F. (32xc2x0 C.) to about 100xc2x0 F. (38xc2x0 C.) and borax is added;
5. Unmodified starch and/or modified starch and/or dextrin is added, and waterproof resin can be added as an option, to make a suspended phase (also called a suspended starch phase) and the contents are mixed for from about 3 to 25 minutes;
6. The contents of the primary mixer are gradually added to the secondary mixer with continuous mixing. This step typically is carried out over a period of about 5 to 20 minutes.
The corn fiber which is added to the primary mixer can be any kind of corn fiber. Suitable fibers include crude fiber, typically described as feed, and more finished products such as dietary corn fiber which is made for human consumption. Crude fiber or feed generally contains from about 20% to about 40% hemicellulose and dietary corn fiber generally contains from about 50% to about 80% hemicellulose. When dietary corn fiber is employed, the amount used is preferably from about 0.1 to about 5.0 parts per 100 parts of adhesive. The fiber in the adhesive, or in a carrier phase, preferably has a particle size of less than about 0.005 inch, a geometric mean size of less than about 0.00005 inch and a median size of less than about 0.00005 inch.
The corrugating adhesive of the invention is an aqueous emulsion which comprises water; from about 0.1 to about 20.0%, preferably from about 0.3% to about 10.0%, hemicellulose by weight based on total starch; from about 10 to about 35, preferably from about 18 to about 32, parts per 100 parts of adhesive of a carbohydrate component which is defined herein as comprising unmodified starch and/or modified starch and/or dextrin; and sufficient caustic to attain a pH from about 10 to about 14, preferably from about 12 to about 14.
In the carrier type embodiment, the carrier phase comprises from about 2 to about 6 parts per 100 parts of adhesive of the carbohydrate component and the remaining carbohydrate component is in a suspended phase. When the adhesive is a no-carrier type adhesive, the carbohydrate component is partially swollen to attain a viscosity from about 20 to about 40 seconds, preferably from about 25 to about 35 seconds, as determined by an orifice type viscometer, commonly known as a Stein Hall cup. Finally, in the carrier-no-carrier type embodiment from about 0.05 to about 0.10 parts of the carbohydrate component (based on total starch) is in a carrier phase and the remainder is partially swollen to attain a viscosity from about 30 to about 50 seconds, preferably from about 35 to about 45 seconds, determined in the same manner as for the no-carrier embodiment.
The adhesive of the invention can contain from about 0.2 to about 1 part per 100 parts of adhesive of a boron containing compound. A waterproofing or water resistant resin also can be incorporated in the adhesive in an amount from about 0.5 to about 5 parts per 100 parts of adhesive.
In a preferred embodiment, the adhesive is a carrier type or carrier-no-carrier type adhesive and the hemicellulose is extracted from corn fiber in situ during preparation of the carrier phase.
The carrier type corrugating adhesive composition of the invention is made by separately preparing two intermediates and then combining them. One intermediate is called the carrier phase and the other is called the suspended starch phase.
The carrier phase is made by admixing the hemicellulose and the carbohydrate component with water. These components can be added to the water together or in any order. The amount of hemicellulose which is added is from about 0.4 to about 80 parts, preferably from about 1.2 to about 40 parts, per 100 parts of carrier phase or from about 0.1 to about 20.0 parts per 100 parts of total starch in the adhesive. The carbohydrate component is added in an amount from about 10 to about 25 parts per 100 parts of carrier phase or from about 2 to about 6 parts per 100 parts of adhesive.
In a preferred embodiment of the invention, the hemicellulose from corn fiber is employed in a carrier type adhesive. The preferred corn fiber is dietary corn fiber containing from about 50 to about 80% hemicellulose and it can be prepared according to U.S. Pat. Nos. 4,994,115 or 5,073,201. The corn fiber containing hemicellulose is admixed with the other ingredients in the preparation of the carrier phase. The components are continuously mixed and heated to a temperature from about 115xc2x0 F. (46xc2x0 C.) to about 180xc2x0 F. (82xc2x0 C.). Sufficient caustic is then added to provide an alkaline pH while continuing mixing and maintaining heating. If the hemicellulose is added in the form of corn fiber, mixing and heating are continued for a sufficient time to extract hemicellulose in situ from the corn fiber. The pH should exceed about 10 and preferably will exceed about 12. The sufficient time and temperature will generally be from about 10 to about 40 minutes at from about 115xc2x0 F. (46xc2x0 C.) to about 180xc2x0 F. (82xc2x0 C.). Lower temperatures generally correspond with longer times.
The preferred fiber of the present invention has a particle size in the adhesive of less than about 0.005 inch, a geometric mean size of less than about 0.00005 inch and a median size of less than about 0.00005 inch.
Following the removal of heat, mixing can be continued to allow uniform cooling. Water can be added at this stage to speed cooling.
The suspended starch phase is made by admixing the carbohydrate component with heated water. The water is heated at a temperature from about 70xc2x0 F. (21xc2x0 C.) to about 105xc2x0 F. (41xc2x0 C.) and is mixed continuously. The carbohydrate component is added in an amount from about 10 to about 25 parts per 100 parts of the corrugating adhesive, and preferably in an amount from about 17 to about 20 parts per 100 parts of the adhesive.
A boron containing compound such as boric acid or a boric acid salt must be added at some stage in the process to improve tack or stickiness. It may be added in the preparation of the carrier phase, the suspended starch phase, or in both phases. When a portion of the boron containing compound is added during the preparation of the carrier phase, it also improves the viscosity stability of the final adhesive product. Preferably, however, most of the boron containing compound is added to the suspended starch phase.
When the boron containing compound is added during the preparation of the carrier phase, it is added in an amount from about 0.03 to about 1 part per 100 parts of carrier phase or in an amount from about 0.01 to about 0.3 parts per 100 parts of adhesive. The boron containing compound added to the suspended starch phase is added in an amount to bring the total boron containing compound up to from about 0.2 to about 1 part per 100 parts of the adhesive.
The carrier phase is gradually added to a tank containing the suspended starch phase with continuous mixing of the contents of the tank. The temperature of the tank contents is maintained at from about 70xc2x0 F. (21xc2x0 C.) to about 105xc2x0 F. (41xc2x0 C.) during the gradual addition. The amount of the carrier phase added is from about 15 to about 50 parts per 100 parts of adhesive.
A waterproofing or water resistant resin may be added to the suspended starch phase tank at any time in an amount from about 0.5 to about 5 parts per 100 parts of adhesive. It is preferable to add the resin following completion of all of the other adhesive preparation steps.
The individual components of the carrier type adhesive composition of the invention are described in detail below along with descriptions of their preferred embodiments.
Hemicellulose
Hemicellulose is a natural polymer present in many living plants and it can be derived from various sources for use according to the invention. Its name is derived from its close association with cellulose in the plant tissue and once it was thought to be a precursor to cellulose synthesis. It is a relatively strong material that resists breakdown through mechanical shearing and the subsequent loss of viscosity. It is completely compatible with starch and reacts positively with boron compounds to yield boron-oxygen bridged structures common to polyhydroxy compounds in corrugating adhesives. Hemicellulose can be partially hydrolyzed if desired to reduce its molecular weight and enhance its viscosity characteristics.
It is possible to extract hemicellulose from corn fiber using an alkaline medium such as calcium hydroxide solution or dilute alkali. However, this step can be eliminated in preparing corrugating adhesives because they are prepared in an alkaline medium. Therefore the hemicellulose can be extracted in situ without the need for a prior extraction-purification step.
Hemicellulose is based on a 5-member xylan (pentosans) structure having additional side chains such as arabano-xylan and glucuronic acid. The skeleton configuration is beta, 1, 4 linkages similar to those found in cellulose. Although its molecular structure is different from starch and cellulose, the key to its similarity is the many functional hydroxyl groups available for bonding. The structure for hemicellulose can vary substantially, but a generalized form of the structure is illustrated below: 
The pericarp or hull of a corn kernel is a particularly good source of hemicellulose because it is high in hemicellulose content, the hemicellulose is readily extracted from it and it is abundantly available. One of the largest, potential, uncontaminated (i.e., not detrimental to the corrugating process) sources of hemicellulose is corn fiber produced by the corn wet milling industry. A preferred type of corn fiber is dietary corn fiber which is sold under the designation PEERLESS(copyright) corn fiber by the Corn Products Unit of CPC International Inc., P. O. Box 8000, Englewood Cliffs, N.J. 07632 USA.
Hemicellulose responds favorably to the addition of boron compounds. The boron bridging mechanism that is common to polyhydroxyl compounds, seems to be another positive attribute of the hemicellulose in starch-based corrugating adhesives.
Carrier Phase Starch or Dextrin
While unmodified starch or dextrin can be used to make the carrier phase of the present invention, modified starch is preferred because it enables the dissolution of substantially more solids (up to twice the amount attainable with unmodified starch) in the adhesive composition and therefore contributes to the development of better green bonding and adhesive characteristics. The reason is that modified starches, when pasted in water, are less viscous than their unmodified counterparts, and as a consequence they can xe2x80x9ccarryxe2x80x9d more ungelatinized starch at practical viscosities.
The modified starch which is used in accordance with the present invention can be mechanically, chemically or heat modified. Compared to unmodified starches, modified starches frequently possess superior physical properties such as increased solubility, better film forming, increased whiteness, improved gel strength, viscosity stability, increased adhesivity, improved resistance to shear and increased resistance to freeze-thaw degradation. Starches derived from other genetic forms of corn, such as high amylose and waxy corn as well as sorghum varieties would also be suitable for such applications. Suitable chemically modified starches include modified oxidized starch such as hypochlorite-oxidized starch, acid-thinned starch, ethylated starch, cross-bonded starch and others which have reduced molecular weight, higher fluidity and/or functional sub groups.
Examples of chemically modified starches which can be used in the invention and are commercially available are SUREBOND(copyright) or STABLEBOND(copyright) modified starches which have residual carboxyl functionality and extreme uniformity and are sold by the Corn Products Unit of CPC International Inc., P. O. Box 8000, Englewood Cliffs, N.J. 07632 USA.
Unmodified starches which can be used in the carrier phase are the same as those described in more detail below in the discussion of the suspension phase starch.
The dextrins which can be used in the carrier phase are prepared by heating starch under various conditions as more fully described in Whistler, R. L. et al., Starch: Chemistry and Technology, 2nd edition, chapter xx, pages 596-607, Academic Press, Inc. (New York, 1984).
Suspension Phase Starch
The unmodified starch used in the suspension phase is a commodity chemical produced from the root, stem or fruit from a number of plants. It is a high molecular weight carbohydrate polymer which is comprised of linear and branched polysaccharide polymers. Modified starch and/or dextrin can be used as the suspension phase starch, but unmodified starch is more economical.
Boron Containing Compound
Any boron containing compound having free hydroxyl groups attached to the boron atoms can be used. The most commonly used compounds are commercial boric acid (ortho boric acid, H3BO3 and its hydrated forms H3BO3.xH2O) and borax (sodium tetraborate decahydrate, Na2B4O7.10H2O and other hydrate and anhydrous forms).
Caustic
Any strong base can be used, but the preferred bases are alkali metal hydroxides. The most preferred are sodium and potassium hydroxide.
Waterproofing/Water Resistance
Starch-based corrugating adhesives prepared with hemicellulose have excellent waterproof characteristics compared with the same adhesive made without hemicellulose. If more enhanced waterproofing or water resistance is desired, however, conventional resins which are employed in the corrugating industry can be used.
Preferred waterproofing or water resistant resins include those which upon heating in basic media generate cross-linking species which react and cross-link with any available hydroxyl group in the starch, polyvinyl alcohol, hemicellulose or cellulose molecules. The cross-linking action reduces the hydrophilic nature and water-solubility of the starch, hemicellulose, and other polyhydroxy molecules by effectively removing the availability of hydroxyl groups to water and by developing hydrophobic, aliphatic cross-linking moieties. Condensation products from the reaction of a ketone and an aldehyde compound are suitable. These resins are characterized as polyether polymers, but can contain a variety of other monomers such as urea, melamine, and the like. The most preferred resins are acetone-formaldehyde resins, acetone-urea-formaldehyde resins and acetone-melamine-formaldehyde resins comprising about 1.5-30% by weight acetone, about 5-50% by weight formaldehyde and about 0-15% of a third monomer. A commercially available cross-linking resin which is suitable for use in the present invention is the ASTROMEL(copyright) series of resins manufactured by Astro Industries, Inc., 114 Industrial Boulevard, P. O. Box 2559, Morganton, N.C. 28655 USA.